Hemispherectomy involves disconnecting one side of the brain (that is, one cerebral hemisphere) from the rest of the brain. In years past, surgeons usually removed a large part of the hemisphere. The name that resulted—anatomic hemispherectomy, cutting out a hemisphere—is still used. Today, however, much less tissue is removed. Instead, the surgeon mostly just cuts the connections from one hemisphere to the rest of the brain so that seizures arising in that hemisphere have nowhere to go. This procedure is called "functional hemispherectomy." There are several variations in the procedure.

Hemispherectomy is only considered for patients (usually children) who have severe epilepsy with seizures arising from only one side of the brain. It is performed only if that hemisphere is already functioning very poorly (because of both prior injury and continued seizure activity), so the patient loses little or no additional function.

The results are quite good. More than 80% of patients experience complete or nearly complete seizure control. The outlook for seizure control is less good if the patient has a progressive disorder such as Rasmussen's syndrome.